The present disclosure relates generally to testing and evaluation of subterranean formation fluids, and, more particularly, to methods and apparatuses for evaluating fluid sample contamination by using multiple sensors.
To evaluate prospects of an underground hydrocarbon reserve, a representative sample of the reservoir fluid may be captured for detailed analysis. A sample of the formation fluids may be obtained by lowering a sampling tool having a sampling chamber into the wellbore on a conveyance such as a wireline, slick line, coiled tubing, jointed tubing or the like. When the sampling tool reaches the desired depth, one or more ports are opened to allow collection of the formation fluids. The ports may be actuated in variety of ways such as by electrical, hydraulic or mechanical methods. Once the ports are opened, formation fluids travel through the ports and a sample of the formation fluids is collected within the sampling chamber of the sampling tool. After the sample has been collected, the sampling tool may be withdrawn from the wellbore so that the formation fluid sample may be analyzed.
Fluid analysis is possible using pumpout formation testers that provide downhole measurements of certain fluid properties and enable collection of a large number of representative samples stored at downhole conditions. The accurate determination of the fluid properties and contamination while sampling with a wireline pumpout formation tester is the primary objective for obtaining representative fluid samples with minimum rig time. This is an important component of the formation evaluation system established by the oil industry, especially for high-profile and offshore wells. During drilling operations, a wellbore is typically filled with a drilling fluid (“mud”), which may be water-based or oil-based. The mud is used as a lubricant and aids in the removal of cuttings from the wellbore, but one of the most important functions of the mud is well control. Hydrocarbons contained in subterranean formations are contained within these formations at very high pressures. Standard overbalanced drilling techniques require that the hydrostatic pressure in the wellbore exceed the formation pressure, thereby preventing formation fluids from flowing uncontrolled into the wellbore. The hydrostatic pressure at any point in the wellbore depends on the height and density of the fluid column of mud above that point. A certain hydrostatic pressure is desired in order to offset the formation pressure and prevent fluid flow into the well. Thus, it is well known in the art to control the mud density, and it is often necessary to use high density “heavy” mud to achieve a desired hydrostatic pressure.
When the hydrostatic pressure of the mud is greater than the pressure of surrounding formation, drilling fluid filtrate will tend to penetrate the surrounding formation. Thus, the fluid in the formation close to the wellbore will be a mixture of drilling fluid filtrate and formation fluid. The presence of fluid filtrate in the formation can interfere with attempts to sample and analyze the formation fluid. As a fluid sample is drawn from the formation at the wall of the wellbore, the first fluid collected may comprise primarily drilling fluid filtrate, with the amount of filtrate in the mixture typically decreasing as collected volume increases.
Early formation testing tools were designed to draw in a fixed volume of fluid and transport that volume to the surface for analysis. It was soon realized that the fixed volume was not sufficient to collect a reasonable sample of formation fluid because the sample would be primarily drilling fluid filtrate. To solve this problem, formation testing tools were developed which were able to continuously pump fluid into the testing tool so that sample collection could be controlled by the operator. Using these types of tools, the operators attempt to avoid collecting filtrate in the fluid sample by pumping for a period of time before collecting fluid sample. Therefore, it is important to determine the quality of the fluid sample in-situ, with the formation tester still in the well, in order to increase the efficiency and effectiveness of sample collection.
While embodiments of this disclosure have been depicted and described and are defined by reference to exemplary embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.